In traditional printing approaches (such as off set, letterpress and gravure), as well as in electronic printing approaches (such as thermal, ink jet and laser or LED electrophotographic), there is a need to register the different color separation planes (e.g. cyan, magenta, yellow and black) that combine to form the final multicolor image. The problems that can create non-register are many and varied, depending upon the specific structures and procedures that are used in each different printing approach. For example, in multicolor ink jet printing, incorrect registration of the different color separation planes can be caused by mechanical positioning differences between actual and nominal positions (in the x or y direction) of the different-color print heads and by differences in the time of drop flight between the different-color print heads (e.g. caused by ink pressure or ink viscosity differences between those print heads.
The usual approach for correcting such errors is to visually inspect the registration of the composite multicolor image and make corrections until registration is judged acceptable. Even with specially printed out test patterns, the registration detection/correction procedure is time consuming and difficult. Moreover, the detection/correction procedure must be done by a very experienced technician, in contrast to an office environment user or "key operator".
U.S. Pat. No. 4,675,696 describes a color ink jet printer in which vertical and horizontal misregistrations of the different color separation planes are automatically detected and corrected by the printer. To implement this approach, different color test bands are printed in adjacent relations; then, the overlap or spacing between, and/or vertical offsets of, the bands are optically detected and stored in a memory system of the printer. Appropriate corrections are computed and used to electro-mechanically or electrically adjust the relative positions of the different-color separation planes printed by the print heads. This approach is conceptually elegant; however, it requires a complex optical detection system. Also, it can be subject to errors in accurately detecting the precise degree of misregistration, particularly in band overlap conditions. For example, the '969 systems relies on certain assumptions as to the "darkness" of each color and the degree of spreading of each printed test pattern portion. If papers or ink components of a system differ from the assumed normal parameters, errors in registration and correction will occur.